In recent years, partly on account of the higher power levels that have become possible, there has been a remarkable growth in the use of laser beam machines which employ fiber lasers. However, the resonance state of the laser light source within a laser beam machine is destabilized by the entry of outside light, disturbing the oscillation state. Disruption of the oscillation state is particularly severe when the light that has been generated is reflected by intermediate optics and returns to the light source. To keep this from happening, an optical isolator is generally provided just in front of the light source, for example.
Optical isolators are made of a Faraday rotator, a polarizer situated on the input side of the Faraday rotator, and an analyzer situated on the output side of the Faraday rotator. The Faraday rotator is used by applying a magnetic field parallel to the propagation direction of light, at which time a polarized component of light, whether traveling forward or backward through the Faraday rotator, rotates only in a fixed direction. In addition, the Faraday rotator is adjusted to a length such that the polarized component of light rotates exactly 45°. When the polarizer and analyzer planes of polarization are offset by 45° in the direction of rotation by forward-traveling light, polarized light traveling forward coincides with the polarizer position and with the analyzer position and thus passes through each. By contrast, polarized light traveling backward from the analyzer position rotates 45° in the opposite direction from the direction of angle offset by the polarizer plane of polarization that is offset 45°. As a result, the returning light has a plane of polarization at the polarizer position that is offset 45°−(−45°)=90° with respect to the polarizer plane of polarization, and thus cannot pass through the polarizer. Hence, the optical isolator functions by allowing forward-traveling light to pass through and exit therefrom and by blocking backward-traveling return light.
Materials hitherto know to be capable of use as the Faraday rotator in optical isolators include TGG crystals (Tb3Ga5O12) and TSAG crystals (Tb(3-x)Sc2Al3O12) (JP-A 2011-213552 (Patent Document 1) and JP-A 2002-293693 (Patent Document 2)). TGG crystals have a relatively large Verdet constant of 40 rad/(T·m), and today are widely used in standard fiber laser systems. TSAG crystals have a Verdet constant which is reportedly about 1.3 times that of TGG crystals and is likewise a material used in fiber laser systems.
In addition, JP-A 2010-285299 (Patent Document 3) discloses a single crystal or ceramic composed primarily of the oxide (TbxR1-x)2O3, wherein 0.4≤x≤1.0 and R is selected from the group consisting of scandium, yttrium, lanthanum, europium, gadolinium, ytterbium, holmium and lutetium. Oxides composed of these constituents have a Verdet constant of 0.18 min/(Oe·cm) or more, with the largest Verdet constant mentioned in the examples provided therein being 0.33 min/(Oe·cm). The same document also mentions, in the text thereof, a Verdet constant for TGG of 0.13 min/(Oe·cm). Hence, the difference between the Verdet constants for both is 2.5-fold.
An oxide composed of substantially similar components is disclosed in JP-A 2011-121837 (Patent Document 4) as well, where it is mentioned that this oxide has a larger Verdet constant than a TGG single crystal.
When, as in Patent Documents 3 and 4 above, an optical isolator having a large Verdet constant is obtained, the total length required for 45° rotation can be shortened, which is desirable in that it makes a smaller optical isolator possible.
Although the (TbxR1-x)2O3 oxides disclosed in Patent Documents 3 and 4 do indeed have very large Verdet constants which are 1.4 to 2.5 times as large as those of the TGG crystals disclosed in Patent Document 1 and the TGG crystals mentioned in the text of Patent Document 3, these oxides end up slightly absorbing fiber laser light in the wavelength range of 0.9 to 1.1 μm where they are expected to be used. With fiber lasers in recent years becoming increasingly high-powered, even when a laser is equipped with an optical isolator having only slight absorption, this leads to deterioration in beam quality on account of a thermal lens effect.
One material that has a very large Verdet constant per unit length is iron (Fe)-containing yttrium iron garnet (YIG) single crystals (JP-A 2000-266947 (Patent Document 5)). However, iron has a large light absorption at a wavelength of 0.9 μm, which absorption affects optical isolators used in the wavelength range of 0.9 to 1.1 μm. This makes optical isolators that use such yttrium iron garnet single crystals very difficult to employ in fiber laser systems where the trend is clearly toward higher power levels.
Hence, there exists a desire for an entirely new material that has a larger Verdet constant than TGG crystals (Tb3Ga5O12) and TSAG crystals (Tb(3-x)Sc2Al3O12), and that does not absorb fiber laser light in the wavelength range of 0.9 to 1.1 μm.
Candidates for such a material include oxides having a pyrochlore-type crystal structure. Pyrochlore-type crystals which have an A2B2O7 crystal structure and for which the ratio between the radii of A ions and B ions falls within a fixed range are known to have a cubic structure. Being able to select a material having a crystal structure that is cubic would make it possible to produce a material which, not only as a single crystal, but even as a ceramic body, has a high transparency and thus could be employed as various types of optical materials.
JP-A 2005-330133 (Patent Document 6) discloses, as examples of such pyrochlore-type materials, cubic titanium oxide pyrochlore sintered bodies characterized in that they are formed by sintering an electron-conducting ceramic powder which is, of the cubic titanium oxide pyrochlores having a rare-earth element RE at the A sites, a complex oxide RE2−xTi2O7-δ wherein the element RE at the A sites is one, two or more of the elements lutetium, ytterbium, thulium, erbium, holmium, yttrium, scandium, dysprosium, terbium, gadolinium, europium, samarium and cerium, and which has a non-stoichiometric amount x of the A-site element RE set within the range0<x<0.5according to the A-site element RE, and subsequently subjecting the sintered powder to reduction treatment. Because the intended application for this art is an electron-conducting ceramic, no mention is made of the transparency of this sintered body. It is known, among those skilled in the art, that normal sintering alone usually yields an opaque sintered body, and so the materials described in Patent Document 6 presumably cannot be used in optical material applications. However, the fact that terbium-containing titanium oxide pyrochlore can have a cubic crystal structure has been disclosed in this publication (Patent Document 6).
Yet, it was separately known before this that a cubic crystal structure is not possible in a simply terbium-doped silicon oxide (see “Rare earth disilicates R2Si2O7(R═Gd, Tb, Dy, Ho): type B,” Z., Kristallogr., Vol. 218, No. 12, 795-801 (2003) (Non-Patent Document 1)).
Also, the fact that certain rare earth-hafnium oxides, although containing no terbium whatsoever, assume a cubic pyrochlore structure and have translucency was disclosed at about the same time (“Fabrication of transparent La2Hf2O7 ceramics from combustion synthesized powders,” Mat. Res. Bull. 40(3), 553-559 (2005) (Non-Patent Document 2)).
In addition, JP-A 2010-241677 (Patent Document 7) discloses an optical ceramic which is a polycrystalline, transparent optical ceramic wherein at least 95 wt %, and preferably at least 98 wt %, of the individual crystals have a cubic pyrochlore or fluorite structure and which contains the stoichiometric compoundA2+xByDzE7 Here, when −1.15≤x≤0, 0≤y≤3 and 0≤z≤1.6, 3x+4y+5z=8. Also, A is at least one trivalent cation selected from the group of rare-earth metal oxides, B is at least one tetravalent cation, D is at least one pentavalent cation, and E is at least one divalent anion. In this optical ceramic, A is selected from among yttrium, gadolinium, ytterbium, lutetium, scandium and lanthanum, and B is selected from among titanium, zirconium, hafnium, tin and germanium. This publication confirms that, in spite of containing no terbium whatsoever, titanium oxide, zirconium oxide, hafnium oxide, tin oxide and germanium oxide containing several types of rare earths can form an at least 98 wt % cubic pyrochlore structure.